Subject to change without notice
32
threshold increases below and above the frequency
limits mentioned in the data sheet. This filter cuts off
both the DC content of the trigger signal and the lowest
frequency range.
DC:
In this coupling mode the trigger signal is coupled gal-
vanically to the trigger unit. Therefore there is no low
frequency limit.
DC triggering is recommended if the signal is to be trig-
gered with quite slow processes or if pulse signals with
constantly changing pulse duty factors have to be dis-
played.
HF:
In this coupling mode the transmission range equals a
high pass filter. It cuts off the DC content of the trigger
signal and the lower frequency range.
NR:
Decreases the maximum trigger bandwidth for high
frequency signals. As in DC coupling condition, lower
frequency signal parts are not affected.
LF:
The LF coupling (low pass filter function) is often more
suited for low-frequency signals than the DC coupling,
because the (white) noise in the trigger voltage is
strongly suppressed. So jitter or double-triggering of
complex signals is avoidable or at least reduced, in
particular with very low input voltages.
TV-L:
The built-in active TV-Sync-Separator provides the
separation of line sync pulses from the video signal.
Even distorted video signals are triggered and dis-
played in a stable manner. This mode is described
under paragraph
“Triggering of video signals”
.
TV-F:
The built-in active TV-Sync-Separator also provides the
separation of frame sync pulses from the video signal.
Even distorted video signals are triggered and
displayed in a stable manner. This mode is described
under paragraph
“Triggering of video signals”
.
Triggering of video signals
In TV-L and TV-F trigger coupling mode the instrument is
automatically set to automatic triggering and the trigger point
indicator is switched off. As only the separated synchro-
nization pulses are used for triggering the relationship bet-
ween the displayed signal and the trigger signal is lost. In TV-
F mode interference may occur if chopped DUAL mode is
chosen or the readout is active. Video signals are triggered in
the automatic mode. The internal triggering is virtually
independent of the display height, but the sync pulse must
exceed 0.5div height.
The polarity of the synchronization pulse is critical for the
slope selection. If the displayed sync pulses are above the
picture (field) contents (leading edge positive going), then
the slope setting for positive going edges must be chosen.
In the case of sync pulses below the field/line, the leading
edge is negative and consequently the slope selection must
be set for falling edges. Since the invert function may cause
a misleading display, it must not be activated.
On the 2ms/div setting and field TV triggering selected, 1
field is visible if a 50 fields/s signal is applied. If the hold off
control is in fully ccw position, it triggers without line
interlacing affects caused by the consecutive field. More
details in the video signal become visible if in alternate or B
time base mode the B time base speed is increased above
the A time base speed. If the time base is set to 5ms/div 2.5
frames are visible. The time delay for the B time base then
can be used to select any line(s) to be displayed expanded
on the screen. The influence of the integrating network which
forms a trigger pulse from the vertical sync pulses may
become visible under certain conditions. Due to the integra-
ting network time constant not all vertical sync pulses starting
the trace are visible.
On the 10µs/div setting and line TV triggering selected,
approx. 1
½
lines are visible. Those lines originate from the
odd and even fields at random. The sync-separator-circuit
also operates with external triggering. It is important that
the voltage range (0.3V
pp
to 3V
pp
) for external triggering should
be noted. Again the correct slope setting is critical, because
the external trigger signal may not have the same polarity or
pulse edge as the test signal displayed on the CRT. This can
be checked, if the external trigger voltage itself is displayed
first (with internal triggering).
In most cases, the composite video signal has a high DC
content. With constant video information (e.g. test pattern
or color bar generator), the DC content can be suppressed
easily by AC input coupling of the oscilloscope amplifier. With
a changing picture content (e.g. normal program), DC input
coupling is recommended, because the display varies its
vertical position on screen with AC input coupling at each
change of the picture content. The DC content can be
compensated using the
Y-POS.
control so that the signal
display lies in the graticule area. Then the composite video
signal should not exceed a vertical height of 6div.
Line / Mains triggering (~)
The instrument specific information regarding this mode is
part of the section
“Controls and Readout”
paragraph
TRIG.
MODE (27)
.
A voltage originating from mains/line (50 to 60Hz) is used for
triggering purposes if this mode is selected. This trigger mode
is independent of amplitude and frequency of the Y signal
and is recommended for all mains/line synchronous signals.
This also applies within certain limits, to whole number
multiples or fractions of the line frequency. Line triggering
can also be useful to display signals below the trigger
threshold (less than 0.5div). It is therefore particularly suitable
for measuring small ripple voltages of mains/line rectifiers or
stray magnetic field in a circuit. In this trigger mode the slope
direction pushbutton selects the positive or negative portion
of the line/mains sine wave.
The trigger level control can be used for trigger point adjust-
ment. Magnetic leakage (e.g. from a power transformer) can
be investigated for direction and amplitude using a search or
pick-up coil. The coil should be wound on a small former
with a maximum of turns of a thin lacquered wire and
connected to a BNC connector (for scope input) via a shielded
cable. Between cable and BNC center conductor a resistor
of at least 100
Ω
should be series-connected (RF de-coupling).
Often it is advisable to shield statically the surface of the
coil. However, no shorted turns are permissible. Maximum,
minimum, and direction to the magnetic source are detectable
at the measuring point by turning and shifting the coil.
Alternate triggering
This trigger mode can be selected in
DUAL
mode by pressing
and holding the
TRIG.
pushbutton
(24)
if the preconditions
are met (please note “Controls and readout”). In the case of
chopped DUAL mode, selecting alternate trigger mode
automatically sets the instrument to alternate DUAL mode.
If the trigger coupling is set to TV-L or TV-F, selecting alternate
triggering automatically results in AC coupling conditions.
Under line/mains triggering condition alternate triggering can
not be chosen. Thus only the following trigger coupling modes
Triggering and time base
